1. Field of the Invention
The present invention relates to digital signal transmission, more particularly, to an error rate reduction scheme for digital signal transmission, especially suitable for digital signal transmission based on the High-Definition Multimedia Interface (HDMI) standard or the Unified Display Interface (UDI) standard.
2. Description of the Related Art
As is well known in the art, digital consumer appliances are often designed to support the HDMI standard for digital signal transmission. For example, hard disc recorders often transmit digital signals by using the HDMI standard, and digital televisions often support the HDMI standard for reception of digital signals. In addition, the UDI standard, which is developed as a modified version of the HDMI standard optimized for personal computer monitors, is widely available for general users.
As defined in the High-Definition Multimedia Interface Specification Version 1.3, an HDMI system transmits digital video data across TMDS (Transmission Minimized Differential Signal) channels: three of which are data channels transmitting differential data signals, and one of which is a clock channel transmitting differential clock signals. The allowed maximum clock rate defined in the TMDS standard is 340 MHz, and the allowed maximum transmission band is 10.2 Gbps. Additionally, the HDMI standard defines “data island periods” provided within the blanking periods of the digital video data. The “data island periods” are used to transmit digital audio data or other auxiliary data. The transmitting side may attach error correction codes with the data transmitted during a data island period. The attached error correction codes are used for error correction on the receiving side.
The HDMI standard supports the CEC (consumer electronic control), which is a protocol for remote control of a specific electronic appliance from another electronic appliance; the CEC is implemented as one of the functions of HDMI interface. Additionally, the HDMI standard supports EDIDs (extended Display Identification). In transmitting digital signals to a display device, the transmitting side receives an EDID from the display device and generates the digital signals to be transmitted in accordance with the data reception specification identified by the EDID.
As described above, the HDMI standard is directed to high-frequency large-amount data transmission over a HDMI cable connected between transmitting and receiving sides. The frequency used in the HDMI-based data transmission is extremely high, ten times as high as those used in other digital interfaces used in conventional electric appliances. Therefore, the HDMI standard imposes severe requirements on cables connected between HDMI devices to avoid signal quality deterioration; high quality cables are necessary for HDMI data transmission. For example, HDMI cables are often provided with gold-plated contacts and/or an equalizer. This results in an undesirable increase in the cost. Recently, low quality HDMI cables which are not subjected to a certification test are commercially accepted; however, a low quality HDMI cable may cause errors in audio/video data transmission.
The data errors in audio/video data transmission caused by a low quality HDMI cable include two modes: One error mode is excessive signal attenuation over the HDMI cable, which occurs especially when cheap materials are used for metal contacts and copper wire for cost reduction. The excessive signal attenuation may reduce the voltage amplitude of a signal to be transmitted below a value required by the HDMI standard, and this may cause unsuccessful data reception due to the insufficient voltage amplitude on the receiving side. Another error mode is excessive signal skew caused by uneven lengths of signal lines. The cost reduction in cable manufacture may result in uneven lengths of signal lines within a HDMI cable, causing skew among signals transmitted over the HDMI cable. When signal skew among three data channels and one clock channel exceed maximum skew required by the HDMI standard, the receiving side may unsuccessfully receive data at regular timings.
A user who purchases such a low quality cable and uses the low quality cable for HDMI device connection may suffer from a problem that the receiving side device (such as a digital television) unsuccessfully receive audio and video signal. Such user may misunderstand that the problem results from a defect of the HDMI device, not the low quality MDMI cable. As a result, the use of a low quality cable may cause disadvantage to HDMI device manufacturers. Therefore, further improvement in the quality of signals generated by the transmitting side is required in electric consumer appliances.
Japanese Laid Open Patent Application No. JP-A 2003-259241 discloses a video processing apparatus for error rate reduction. FIG. 1 illustrates a configuration of the disclosed video processing apparatus. The disclosed video processing apparatus is provided with a transmitting apparatus 100 and a receiving apparatus 200 which are connected through interfaces 402 and 403. The transmitting apparatus 100 includes a video signal source 101, an audio signal source 102, an audio/video multiplexer 103, a transmitter circuit 104, a scan converter 106 and a resolution controller 107. The receiving apparatus 200 includes a receiver circuit 201, an audio/video demultiplexer 202, an adder 203, a display device 204, a D-A converter/audio processor 205, a speaker 207, a switch controller 208, an OSD (on-screen display) circuit 209, first and second EDID circuits 211 and 212, switches 213 and 214, a scan converter 215, a memory device 216.
The receiving apparatus 200 receives a digital video signal from the transmitting apparatus 100. The digital video signal received by the receiving apparatus 200 is fed to the switch controller 208 through the audio/video demultiplexer 202. The switch controller 208 detects the error rate of the digital video signal. When the detected error rate is higher than a predetermined threshold value, the switch controller 208 controls the switch 213 so as to select the second EDID circuit 212, which contains format data for low resolution. The selected format data are transmitted to the transmitting apparatus 100 through the interface 403. The transmitting apparatus 100 generates a digital video signal having a reduced scanning ruling and/or a reduced horizontal resolution compared to the digital video signal transmitted just before. This allows the transmitting rate of the data transmitted over the cable, resulting in the reduction of the error rate. The video processing apparatus is also configured to inform the user that the noise on the image displayed on the display device 204 is resulting from the cable quality not the device failure by the on-screen display.
However, the video processing apparatus suffers from a drawback that the video image is not displayed with originally desired resolution, because the transmitting apparatus 100 reduces the resolution of the video image for error rate reduction. The image quality deterioration is not avoided, when the user uses a low quality HDMI cable.